Lithographic apparatus, device manufacturing method, and device manufactured thereby

ABSTRACT

In a lithographic projection apparatus, a grating spectral filter is used to filter an EUV projection beam. The grating spectral filter is preferably a blazed, grazing incidence, reflective grating. Cooling channels may be provided in or on the rear of the grating spectral filter. The grating spectral filter may be formed of a material effectively invisible to the desired radiation.

BACKGROUND OF THE INVENTION

[0001] This application claims priority to European Patent Application00308903.4, filed Oct. 10, 2000 which is herein incorporated byreference in its entirety.

[0002] 1. Field of the Invention

[0003] The present invention relates generally to a lithographicprojection apparatus and more particularly to a lithographic projectionapparatus including a spectral filter.

[0004] 2. Background of the Related Art

[0005] The term “patterning structure” as here employed should bebroadly interpreted as referring to structure that can be used to endowan incoming radiation beam with a patterned cross-section, correspondingto a pattern that is to be created in a target portion of the substrate;the term “light valve” can also be used in this context. Generally, thepattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). Examples of such patterning structure include:

[0006] A mask. The concept of a mask is well known in lithography, andit includes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support structurewill generally be a mask table, which ensures that the mask can be heldat a desired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired.

[0007] A programmable mirror array. An example of such a device is amatrix-addressable surface having a viscoelastic control layer and areflective surface. The basic principle behind such an apparatus is that(for example) addressed areas of the reflective surface reflect incidentlight as diffracted light, whereas unaddressed areas reflect incidentlight as undiffracted light. Using an appropriate filter, the saidundiffracted light can be filtered out of the reflected beam, leavingonly the diffracted light behind; in this manner, the beam becomespatterned according to the addressing pattern of the matrix-addressablesurface. The required matrix addressing can be performed using suitableelectronic means. More information on such mirror arrays can be gleaned,for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which areincorporated herein by reference. In the case of a programmable mirrorarray, the said support structure may be embodied as a frame or table,for example, which may be fixed or movable as required.

[0008] A programmable LCD array. An example of such a construction isgiven in U.S. Pat. No. 5,229,872, which is incorporated herein byreference. As above, the support structure in this case may be embodiedas a frame or table, for example, which may be fixed or movable asrequired.

[0009] For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table; however, the general principles discussed in such instancesshould be seen in the broader context of the patterning structure ashereabove set forth.

[0010] Lithographic projection apparatus can be used, for example, inthe manufacture of integrated circuits (ICs). In such a case, thepatterning structure may generate a circuit pattern corresponding to anindividual layer of the IC, and this pattern can be imaged onto a targetportion (e.g. comprising one or more dies) on a substrate (siliconwafer) that has been coated with a layer of radiation-sensitive material(resist). In general, a single wafer will contain a whole network ofadjacent target portions that are successively irradiated via theprojection system, one at a time. In current apparatus, employingpatterning by a mask on a mask table, a distinction can be made betweentwo different types of machine. In one type of lithographic projectionapparatus, each target portion is irradiated by exposing the entire maskpattern onto the target portion at once; such an apparatus is commonlyreferred to as a wafer stepper. In an alternative apparatus—commonlyreferred to as a step-and-scan apparatus—each target portion isirradiated by progressively scanning the mask pattern under theprojection beam in a given reference direction (the “scanning”direction) while synchronously scanning the substrate table parallel oranti-parallel to this direction; since, in general, the projectionsystem will have a magnification factor M (generally <1), the speed V atwhich the substrate table is scanned will be a factor M times that atwhich the mask table is scanned. More information with regard tolithographic devices as here described can be gleaned, for example, fromU.S. Pat. No. 6,046,792, incorporated herein by reference.

[0011] In a manufacturing process using a lithographic projectionapparatus, a pattern (e.g. in a mask) is imaged onto a substrate that isat least partially covered by a layer of radiation-sensitive material(resist). Prior to this imaging step, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g., anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4, incorporated herein by reference.

[0012] For the sake of simplicity, the projection system may hereinafterbe referred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens.”Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel, orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Twin stage lithographicapparatus are described, for example, in U.S. Pat. Nos. 5,969,441 and WO98/40791, incorporated herein by reference.

[0013] In a lithographic apparatus the size of features that can beimaged onto the substrate is limited by the wavelength of the projectionradiation. To produce integrated circuits with a higher density ofdevices, and hence higher operating speeds, it is desirable to be ableto image smaller features. While most current lithographic projectionapparatus employ ultraviolet light generated by mercury lamps or excimerlasers, it has been proposed to use shorter wavelength radiation ofaround 13 nm. Such radiation is termed extreme ultraviolet (EUV) or softx-ray and possible sources include, for instance, laser-produced plasmasources, discharge plasma sources, or synchrotron radiation fromelectron storage rings. Apparatus using discharge plasma sources aredescribed in: W. Partlo, I. Fomenkov, R. Oliver, D. Birx, “Developmentof an EUV (13.5 nm) Light Source Employing a Dense Plasma Focus inLithium Vapor”,Proc SPIE 3997, pp. 136-156, 2000; M.W. McGeoch, “PowerScaling of a Z-pinch Extreme Ultraviolet Source”, Proc SPIE 3997, pp.861-866, 2000; and W. T. Silfvast, M. Klosner, G. Shimkaveg, H. Bender,G. Kubiak, N. Fornaciari, “High-power plasma discharge source at 13.5and 11.4 nm for EUV lithography”, Proc SPIE 3676, pp. 272-275, 1999.

[0014] Some extreme ultraviolet sources, especially plasma sources, emitradiation over a wide range of frequencies, even including infrared(IR), visible, ultraviolet (UV) and deep ultraviolet. These unwantedfrequencies will propagate and cause heating problems in theillumination and projection systems and cause unwanted exposure of theresist if not blocked; although the multilayer mirrors of theillumination and projection systems are optimized for reflection of thedesired wavelength, e.g. 13 nm, they are optically flat and have quitehigh reflectivities at IR, visible and UV wavelengths. It is thereforenecessary to select from the source a relatively narrow band offrequencies for the projection beam. Even where the source has arelatively narrow emission line, it is necessary to reject radiation outof that line, especially at longer wavelengths. It has been proposed touse a thin membrane as a filter to perform this function. However, sucha film is very delicate and becomes very hot, 200-300° C. or more,leading to high thermal stresses and cracking, sublimation and oxidationin the high power levels necessary in a lithographic projectionapparatus. A membrane filter also generally absorbs at least 50% of thedesired radiation.

SUMMARY OF THE INVENTION

[0015] One aspect of an embodiment of the present invention provides animproved filter that may be used in a lithographic projection apparatusto select a relatively narrow band of wavelengths from a wide bandsource or to reject unwanted frequencies.

[0016] According to the present invention there is provided alithographic projection apparatus including a radiation system toprovide a projection beam of radiation, a support structure to supportpatterning structure, the patterning structure serving to pattern theprojection beam according to a desired pattern, a substrate table forholding a substrate, a projection system to project the patterned beamonto a target portion of the substrate, and a grating spectral filtercomprised in said radiation system for passing radiation of desiredwavelengths to form said projection beam and for deflecting radiation ofundesired wavelengths. Embodiments of the grating spectral filter of thepresent invention are more efficient, directing a higher proportion ofthe desired radiation into the projection beam, and more robust thanmembrane filters used in the prior art. In particular, certainembodiments of the grating spectral filter are less prone to thermalradiation because they can reflect rather than absorb the undesiredradiation, because they can be made thicker and because cooling channelscan be integrated or attached to a rear surface thereof. By suitableselection of parameters of the grating filter, such as the line densityand angle of the incident beam, the resolving power, which determinesthe wavelength band passed into the projection beam, can be adjusted asdesired. Further, a reflective, grazing incidence grating filter of thesize necessitated by the beam diameter can be provided more easily thana transmisive filter.

[0017] The grating spectral filter is preferably a blazed gratingbecause such gratings have a high diffraction efficiency. For maximumreflection efficiency, the grating is preferably used at grazingincidence. A laminar grating with a square wave surface profile may alsobe used and can be cheaply manufactured.

[0018] Aspects of embodiments of the present invention also provide adevice manufacturing method comprising providing a substrate that is atleast partially covered by a layer of radiation-sensitive material,providing a projection beam of radiation using a radiation system, usingpatterning structure to endow the projection beam with a pattern in itscross-section, projecting the patterned beam of radiation onto a targetportion of the layer of radiation-sensitive material, and filtering saidprojection beam in said radiation system using a grating spectralfilter.

[0019] Although specific reference may be made in this text to the useof the apparatus according to the invention in the manufacture of ICs,it should be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask,” “substrate” and “targetportion,” respectively.

[0020] In the present document, the terms “radiation” and “beam” areused to encompass all types of electromagnetic radiation, includingultraviolet radiation (e.g., with a wavelength of 365, 248, 193, 157 or126 nm) and extreme ultra-violet (UV) radiation (e.g., having awavelength in the range 5-20 nm).

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention and its attendant advantages will bedescribed below with reference to exemplary embodiments and theaccompanying schematic drawings, in which:

[0022]FIG. 1 depicts a lithographic projection apparatus according to afirst embodiment of the invention;

[0023]FIG. 2 is a diagram used to explain the principle of the presentinvention;

[0024]FIG. 3 is a schematic illustration of a blazed grating useable inthe invention;

[0025]FIG. 4 is a schematic illustration of a laminar grating useable inthe invention; and

[0026]FIG. 5 is a diagram used to explain the cut-off wavelength of agrating filter according to the invention;

[0027]FIG. 6 is a diagram of part of an illumination system according toa third embodiment of the invention;

[0028]FIG. 7 is a diagram of a filter according to a fourth embodimentof the invention;

[0029]FIG. 8 is an enlarged diagram of part of the filter of FIG. 7; and

[0030]FIG. 9 is a diagram of a filter according to a fifth embodiment ofthe invention.

[0031] In the various drawings, like parts are indicated by likereferences.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0032] Embodiment 1

[0033]FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatusincludes:

[0034] a radiation system Ex, IL, for supplying a projection beam PB ofradiation (e.g., EUV radiation), which in this particular case alsocomprises a radiation source LA;

[0035] a first object table (mask table) MT provided with a mask holderfor holding a mask MA (e.g., a reticle), and connected to firstpositioning means PM for accurately positioning the mask with respect toitem PL;

[0036] a second object table (substrate table) WT provided with asubstrate holder for holding a substrate W (e.g., a resist-coatedsilicon wafer), and connected to second positioning means PW foraccurately positioning the substrate with respect to item PL;

[0037] a projection system (“lens”)PL (e.g., a mirror group) for imagingan irradiated portion of the mask MA onto a target portion C (e.g.,comprising one or more dies) of the substrate W.

[0038] As here depicted, the apparatus is of a reflective type (i.e. hasa reflective mask). However, in general, it may also be of atransmissive type, for example (with a transmissive mask).Alternatively, the apparatus may employ another kind of patterningstructure, such as a programmable mirror array of a type as referred toabove.

[0039] The source LA (e.g., a laser-produced or discharge plasma source)produces a beam of radiation. This beam is fed into an illuminationsystem (illuminator) IL, either directly or after having traversedconditioning means, such as a beam expander Ex, for example. Theilluminator IL may comprise adjusting means AM for setting the outerand/or inner radial extent (commonly referred to as σ-outer and σ-inner,respectively) of the intensity distribution in the beam. In addition, itwill generally comprise various other components, such as an integratorIN and a condenser CO. In this way, the beam PB impinging on the mask MAhas a desired uniformity and intensity distribution in itscross-section.

[0040] It should be noted with regard to FIG. 1 that the source LA maybe within the housing of the lithographic projection apparatus (as isoften the case when the source LA is a mercury lamp, for example), butthat it may also be remote from the lithographic projection apparatus,the radiation beam which it produces being led into the apparatus (e.g.,with the aid of suitable directing mirrors); this latter scenario isoften the case when the source LA is an excimer laser. The currentinvention and Claims encompass both of these scenarios.

[0041] The beam PB subsequently intercepts the mask MA, which is held ona mask table MT. Having been selectively reflected by the mask MA, thebeam PB passes through the lens PL, which focuses the beam PB onto atarget portion C of the substrate W. With the aid of the secondpositioning means (and interferometric measuring means IF), thesubstrate table WT can be moved accurately, e.g., so as to positiondifferent target portions C in the path of the beam PB. Similarly, thefirst positioning means can be used to accurately position the mask MAwith respect to the path of the beam PB, e.g., after mechanicalretrieval of the mask MA from a mask library, or during a scan. Ingeneral, movement of the object tables MT, WT will be realized with theaid of a long-stroke module (course positioning) and a short-strokemodule (fine positioning), which are not explicitly depicted in FIG. 1.However, in the case of a wafer stepper (as opposed to a step-and-scanapparatus) the mask table MT may just be connected to a short strokeactuator, or may be fixed.

[0042] The depicted apparatus can be used in two different modes:

[0043] In step mode, the mask table MT is kept essentially stationary,and an entire mask image is projected at once (i.e., a single“flash”)onto a target portion C. The substrate table WT is then shiftedin the x and/or y directions so that a different target portion C can beirradiated by the beam PB.

[0044] In scan mode, essentially the same scenario applies, except thata given target portion C is not exposed in a single “flash.” Instead,the mask table MT is movable in a given direction (the so-called “scandirection”, e.g., the y direction) with a speed v, so that theprojection beam PB is caused to scan over a mask image; concurrently,the substrate table WT is simultaneously moved in the same or oppositedirection at a speed V=Mv, in which M is the magnification of the lensPL (typically, M=¼ or ⅕). In this manner, a relatively large targetportion C can be exposed, without having to compromise on resolution.

[0045] To remove unwanted wavelengths from the output of the radiationsource, a spectral grating filter 100 is placed in the illuminationsystem, conveniently near the source to minimize heat to and in theremainder of the system but at the earliest stage where the filter canbe situated in either a converging or diverging beam. The opening angleof the beam should be limited to have the sufficient wavelengthselectivity. The grating filter may be a reflective grating that willdiffract incident radiation at an angle dependent on wavelength. Thegrating filter 100 is arranged so that the desired radiation enters theremainder of the illumination system while the unwanted radiation isdiffracted at different angles and absorbed by beam blockers.

[0046]FIG. 2 shows a possible arrangement of the grating filter with aconverging beam 10. In the absence of the grating it would be focused ona point behind the grating. It is now incident on the grating under anangle α and the virtual source point is at a distance or behind thegrating. Radiation of the desired wavelength is diffracted into an angleβ into the diffracted beam 20 and converges at a distance or′ from thegrating. This point can be on the mask but can also be a point on aplane in the illumination system from which another optical elementwould focus the beam on the mask.

[0047] The grating may be a blazed grating, such as shown in FIG. 3. Ablazed grating has a sawtooth profile with the grating constant, p,equal to the width of a period of the sawtooth. The blaze angle ba canbe chosen such that the reflection as compared to the sawtooth profileis mirror-like, i.e., the incident and reflected angles α′, β′ measuredfrom the normal n′ to the sawtooth profile are equal. With such anarrangement, a diffraction efficiency of 100% is theoretically possible.A suitable grating of this type is described by Franke et al, J. Vac.Sci. Technol. B 15, p2940 (1997), incorporated herein by reference, andhas a diffraction efficiency of 80% and overall efficiency of about 65%taking into account 80% grazing incidence reflection efficiency.

[0048] Assuming an acceptance angle Δα=0.5°, equivalent to a spot sizeof 4 mm a distance of 458 mm from the grating, and a desired radiationwavelength of 13.5 nm possible arrangements are set out in Table 1below: Line Density (lines/mm) Blaze angle, ba α β λ/Δλ λcutoff (nm) 3001.5 87.063 −84.063 ˜9 14.966 300 2.0 88.674 −84.674 ˜20 14.144 400 1.585.58  −82.58  ˜8 15.166 400 2.0 87.563 −83.563 ˜14.5 14.407 600 1.582.6  −79.6  ˜7.2 15.366 600 2.0 85.366 −81.336 ˜11.4 14.671

[0049] The values for β are based on the convention that negative anglesate the other side of the normal, n, than α. The cutoff value, λcutoff,is not exact but based on the acceptance angle Δα; the longestwavelength ray that will be accepted is that with angles α+Δα/2 andβ+Δβ/2, as shown in FIG. 5, from which the cutoff wavelength isdetermined using the grating equation.

[0050] The resolving power, λ/Δλ, of the grating derives from fivecontributions. Firstly and secondly, the contributions from the entranceand exit slits: $\begin{matrix}{{\frac{\Delta\lambda}{\lambda} = \frac{{{\Delta\alpha} \cdot \cos}\quad \alpha}{Nk\lambda}}{and}} & (1) \\{\frac{\Delta\lambda}{\lambda} = \frac{{{\Delta\beta} \cdot \cos}\quad \beta}{Nk\lambda}} & (2)\end{matrix}$

[0051] derived from Nkλ=sin α+sin β, where N is the line density and kis the order, 1 in the present case. The contribution from Δα isincluded in Table 1 above.

[0052] The third contribution is dependent on the number of linesilluminated and can be neglected in the present application as thenumber of lines illuminated is very large. The fourth and fifthcontributions are dependent on optical aberrations in the system andimperfections in the optical surface of the grating. A suitably highquality system can be selected so that these contributions can beneglected. The total resolution is then the combination of thecontributions from the entrance and exit slits and will be {squareroot}2 times that shown in Table 1 if both contributions are equal.

[0053] As an alternative to the blazed grating, a laminar grating of thetype shown in FIG. 4 may be used. As shown in FIG. 4, the laminargrating has a square wave surface profile with the grating constant, d,equal to one period of the square wave. Such a grating can easily bemade, e.g. using holographic lithography techniques. In such atechnique, the upper surface is polished to a very good surfaceroughness and grooves are etched with the lands defined by a photoresistpattern. Gratings commercially available form a variety of sources aresuitable and can provide a maximum diffraction efficiency of about 40%.

[0054] To cool the mirror in use, cooling channels 101 can be providedon the rear surface of the grating, as shown in FIG. 2, or incorporatedin the body of the grating. A suitable coolant fluid is passed throughthe cooling channels 101 to maintain the mirror at the desiredtemperature. The grating 100 can be made as thick as required, e.g. oforder 50 mm, for the desired mechanical properties and physicalrobustness required in use.

[0055] Laser-produced plasma sources as well as plasma discharge sourcesemit a stream of fast ions and atoms in addition to the desiredradiation. When a membrane is used for wavelength filtering, it alsofunctions to block the ions and atoms. The grating filter of the presentinvention is advantageously combined with a gas-based system forblocking the ions and atoms. This combination allows the debris andradiation filtering to be carried out close to the radiation source,minimizing contamination and heating in the illumination system.

[0056] Embodiment 2

[0057] In a second embodiment of the invention, which may be the same asthe first embodiment of the invention save as described below, thegrating is positioned in a diverging beam so there is a real object andvirtual image. Using a constant line spacing grating, the distances orand or′, the distances from the grating to the object and image, aredifferent. In both the first and second embodiments, a grating with avariable line spacing can be used to alter these distances and make themequal in which case the grating has almost the same properties as aplane mirror. The gratings of both the first and second embodiments canbe combined with another reflector, e.g. a scatter plate or sphericalfocusing mirror, included in the illumination system to have a combinedfunction.

[0058] Embodiment 3

[0059] In a third embodiment of the invention, which may be the same asthe first or second embodiments save as described below, a gratingstructure is applied to a grazing incidence mirror in the illuminationsystem.

[0060] As shown in FIG. 6, radiation from radiation source LA iscollected by collector mirror CM and directed towards grazing incidencereflector 200. The beam directed towards reflector 200 contains both thedesired EUV radiation, e.g. at a wavelength of about 13.5 nm, andundesired radiation at higher wavelengths. If the source LA is, forexample, a laser-produced xenon-plasma, there may be of the order of 10times as much energy in the UV radiation band of 100-200 nm as in thedesired EUV band around 13.5 nm.

[0061] To extract the unwanted longer-wavelength radiation, adiffraction grating structure, such as consisting of a phase grating asshown in FIG. 4, is applied to the reflector 200. The grating isarranged so that the optical path difference (OPD) between rays havingbeen reflected by the “land” L and “groove” G parts of the grating is aninteger multiple of the wavelength of the desired wavelength, i.e.,

OPD=n·λ _(euv)  (3)

[0062] where n is an integer and λ_(euv) the wavelength of the desiredradiation, e.g. 13.5 nm. At the same time, the OPD for the above rays ischosen to be an integer multiple plus half a wavelength in an undesiredradiation range, i.e.

OPD=(m+½)·λ_(ud)  (4)

[0063] where m is an integer and λ_(ud) the wavelength of the undesiredwavelength.

[0064] By satisfying equation (3), the grating does not disturb thedesired radiation which is reflected as intended by reflector 200,effectively all desired radiation is diffracted into the 0^(th) orderbeam. However, the undesired radiation is diffracted as determined bythe grating equation and for wavelengths for which equation (4) isexactly satisfied the diffraction efficiency is at a maximum with allenergy diffracted out of the 0^(th) order beam. In this way, theundesired radiation is spatially separated from the desired radiationforming projection beam PB and can be absorbed by a suitable heat sinkor beam dump 210.

[0065] As an example, the diffractive structure can form a phasegrating, as shown in FIG. 4, with 50% duty cycle, depth h and period pof the order of 100·λ_(euv). The depth h is chosen such that the OPD,given by:

OPD=2h·sin(α)  (5)

[0066] where α is the grazing incidence angle, is 67.5 nm. For a grazingangle α=15°, d is 130 nm, which can be accurately realized. With such agrating, OPD=5·λ_(euv) for λ_(euv)=13.5 nm so that all the desiredradiation goes into the 0^(th)-order beam. For radiation at 135 nm,OPD=½. λ_(ud) so that substantially no radiation around this wavelengthgoes into the 0 ^(th)-order beam and the maximum energy at undesiredwavelength is separated from the desired radiation.

[0067] It should be noted that any imperfection in the grating height,e.g. due to manufacturing tolerances may reduce the amount of desiredradiation going into the 0^(th)-order beam but will not causesignificant undesired energy to go into the 0^(th)-order beam. Anylosses in the desired radiation going into the 0^(th)-order beam willstill be much less than the losses resulting from a metallic absorptionfilter.

[0068] Embodiment 4

[0069] The fourth embodiment is similar to the third embodiment, but thegrating applied to the mirror is constructed from a material having arefractive index close to that of the medium through which theprojection beam travels before it is incident on the grating. For EUVwith an evacuated radiation system, the complex refractive index shouldhave a real part unity and a small imaginary part (low absorption) atthe desired wavelength but a refractive index substantially differentfrom unity at other wavelengths.

[0070] For EUV at 13.5 nm, silicon can be used to form the diffractivestructure as its complex refractive index at that wavelength is givenby:

n=0.9993−0.0018j.

[0071]FIGS. 7 and 8, which is an enlargement of part of FIG. 7, show adiffractive structure comprising protrusions of silicon deposited on agrazing incidence (GI) mirror 300. The EUV radiation at 13.5 nm isessentially unaffected by the diffractive structure 301 and reflectsnormally to form the projection beam PB. The unwanted radiation isdiffracted by the diffractive structure 301 and blocked by spatialfilter 302.

[0072] As best shown in FIG. 8, the diffractive structure is constructedso that the desired radiation PB only passes one period of thediffractive structure. The angle of incidence of the projection beamwith a side surface of the diffractive structure is shown as α′₁. Thisangle should remain small to minimize reflection of the projection beamby a capping layer provided on the silicon diffractive structure 301since cap layer materials tend to have a high reflectivity for grazingincidence angles. Also, the upper surface of the protrusions of thediffractive structure can be angled, as in a blazed grating, so that the0^(th)-order of the unwanted radiation is directed in a differentdirection than the desired radiation. Further, a material with a poorgrazing reflectivity (high transmission) property for the desiredradiation is preferred for the capping layer. Where the desiredradiation is EUV, carbon is suitable.

[0073] The effectiveness of the diffractive structure can be derived asdiscussed below.

[0074] The maxima of the diffracted spectrum can be calculated using:

a(sin γ−sin α)=mλ,  (6)

[0075] where a is the period of the diffractive structure, m is theorder of the maximum, and α, and γ are the angles of the incoming andthe diffracted beam with respect to the normal of the mirror 300 (notwith respect to the diffractive structure), respectively. We note thatexpression (6) is independent of the exact structure of the diffractivestructure.

[0076] As an example (for illustrative purposes only), we take a=2000nm, αhd 1=85°, h_(f)=10 mm, d=20 mm, and l−150 nm, where d is the widthof the opening aperture in spatial filter 302, h_(f) is the distancebetween the diffractive structure and spatial filter 302, and l is thewidth of a protrusion of the diffractive structure. In order to lose thediffracted light (except for the 0^(th)-order), the light has to bedeflected more than the beam 303 in FIG. 7 which is incident on the edgeof the opening aperture of spatial filter 302. For the deflected beam303, the following expression holds: $\begin{matrix}{\gamma_{1} = {{\arctan ( {\frac{d}{2h_{f}} + {\tan \quad \alpha_{1}}} )} = {85.4^{\circ}.}}} & (7)\end{matrix}$

[0077] Combining this expression with expression (6), we can calculatethe minimum wavelength of the radiation that would be deflected morethan beam 303 in FIG. 1:

λ₁=1.2 nm.

[0078] So we can conclude that for a reflection phase grating with apitch of 2000 nm, radiation with a wavelength larger than 1.2 nm will besuppressed by the spatial filter, provided the grating structure is“visible” to that radiation. However, the grating structure is notvisible to the desired radiation because it is made of a materialselected to have a refractive index close to the vacuum value at therelevant wavelengths.

[0079] The height h of the protrusions can be calculated using:$\begin{matrix}{h = {\frac{\alpha - l}{2\tan \quad \alpha_{1}} = {81\quad {{nm}.}}}} & (8)\end{matrix}$

[0080] The maximum path length of the beam in the notch, l_(max) is:$\begin{matrix}{l_{\max} = {\frac{l}{\cos \quad \alpha_{1}} = {\frac{l}{\sin \quad \alpha_{1}} = {151\quad {{nm}.}}}}} & (9)\end{matrix}$

[0081] The influence of the protrusion structure on the desired, e.g.EUV, reflection can be calculated by comparing the transmission of theprotrusion at different positions within a period. The transmission of asilicon layer of thickness d_(Si) is: $\begin{matrix}{t = {\exp - \frac{d_{si}}{\mu}}} & (10)\end{matrix}$

[0082] where μ is the attenuation length of 13.5 nm EUV radiation insilicon (588.2 nm). The maximum absorption in a rectangular protrusionstructure is: $\begin{matrix}{\chi_{A} = {1 - {{\exp ( {- \frac{l_{\max}}{\mu}} )}.}}} & (11)\end{matrix}$

[0083] For the structure as given above, this absorption is equal to23%. For a horizontal protrusion top surface, the fraction of theincident radiation that hits the top of the protrusion is equal tol/a=7.5%. At maximum 2l/a=15% of the EUV radiation will be lost bydiffraction, assuming the radiation impinging on the top surface istotally reflected, and out of phase in the direction of the specularreflection at the mirror surface. So the maximum loss by diffraction is:$\begin{matrix}{\chi_{d} = {\frac{2l}{a}.}} & (12)\end{matrix}$

[0084] The total specular reflection for EUV radiation of the mirrorincluding the diffractive structure is:

R=R _(GI)·(1−χ_(A))·(1−χ_(d))·  (13)

[0085] For a mirror 300 with a 1 nm rms roughness ruthenium layer withthe diffractive structure as given above, the minimum transmission willbe in the order of 61% for the desired EUV radiation.

[0086] Note that the surface roughness of the diffractive structureshould be low in order to have a low stray light level. However, thetoughness criterion is less stringent then for the multilayer mirrorsurface and grazing incidence mirror surfaces. It is comparable to theroughness criterion of EUV filters. This is the main advantage of thediffracted structure made of an EUV invisible material, compared toother spectral purity filters.

[0087] When a converging beam is reflected on the GI mirror, thecalculation as given above should be carried out for the lowest incidentangle α₁ (highest grazing angle). It is possible that a ray passes morethan one notch, resulting in less transmission. As an example, we takea=1000 nm, α₁−70°, and l−150 nm. Then h−155 nm. For incident angles inthe range 70-85°, the average reflection coefficient is 82%, the averagetransmission of the protrusions is 73% (angles above 82° are transmittedthrough 2 notches). The top surfaces are assumed to be tilted to anangle parallel to the average incident angle (77.5°). Then the averageeffective top surface width is 59 nm, resulting in a loss of 12%. Thetotal average transmission is 53%.

[0088] The diffractive structure on the mirror can be produced by rulingwith a diamond tool. It is also possible to produce the diffractivestructure by ion etching of a sinusoidal structure. Producing a blockstructure on a GI-mirror using lithographic techniques, with subsequention etching is also a viable option. The structure as shown in FIGS. 7and 8 is given as an example; other structures are also possible,provided the absorption of EUV radiation is small (path length l_(max)small), and that there is sufficient suppression of the 0^(th)-order fornon-EUV radiation, and minimum diffraction of EUV radiation.

[0089] Embodiment 5

[0090] A fifth embodiment of the present invention is similar to thefourth embodiment but the “invisible” diffractive structure is appliedto a multilayer mirror 400.

[0091]FIG. 9 shows a converging beam 401 incident on a multilayer mirror400 which reflects desired radiation, e.g., EUV at 13.5 nm, to formprojection beam PB. The multilayer mirror is optimized for reflection ofdesired radiation incident at angles between α and α₂. A diffractivestructure that has no effect on the desired radiation, i.e., issubstantially “invisible” to that radiation, is deposited on the mirror.As in the fourth embodiment this may be formed from silicon where thedesired radiation is EUV.

[0092] The diffractive structure diffracts the undesired radiation intobeams 403 away from the desired radiation which are then blocked byspatial filter 402. By giving the diffractive structure a blazedprofile, both the 0^(th)-order and higher orders of the undesiredwavelength can be diffracted away from the desired radiation.

[0093] The effectiveness of the diffractive structure can bedemonstrated as follows and is given for illustrative purposes only:

[0094] The maxima of the diffracted spectrum can be calculated usingexpression (6). As an example, we take a=500 nm, α₁=20°, α₂=6°,h_(f)=100 mm, d=20 mm and d₁=d=20 mm, where distance d₁ as indicated inFIG. 9, is taken identically to the width d for simplicity. In order tolose the diffracted light (except for the 0^(th)-order), the light hasto be deflected more than γ₁ and γ₂ in FIG. 9. For the deflected beams403, the following expressions hold: $\begin{matrix}{{\gamma_{1} = {{\arctan \frac{2d}{h_{f}}} = 21.8^{\circ}}},{and}} & (14) \\{\gamma_{2} = {{\arctan \frac{d}{h_{f}}} = {11.3^{{^\circ}}.}}} & (15)\end{matrix}$

[0095] Using these expressions, and combining them with expression (6),we can calculate the minimum wavelength of the radiation that isdeflected more than γ₁ and γ₂ in FIG. 1:

λ₁=1.47 nm

λ₂=4.58 nm

[0096] So we can conclude that for a reflection phase grating with apitch of 500 nm, radiation with a wavelength of more than 45.8 nm willbe suppressed by the spatial filter.

[0097] The 0^(th)-order is suppressed by for instance choosing a wedgeshape for the diffractive structure. With a wedge shaped structure, theposition of the specular reflection of the diffractive structure can bechanged independently of the first and higher orders. The followingexpressions hold for the beams 404 in FIG. 9. $\begin{matrix}{{\beta_{1} = {{\arctan \frac{d}{h_{f}}} = 11.3^{{^\circ}}}},{and}} & (16) \\{\beta_{2} = {{\arctan \frac{0}{h_{f}}} = {0^{{^\circ}}.}}} & (17)\end{matrix}$

[0098] The 0^(th)-order has to be deflected at least 20°−11.3°8.7° forthe first ray 401 a and 6°−0°=6° for the second ray 401 b towards thenormal to the surface of mirror 400 in FIG. 9. Therefore, a blaze angleba of 8.7°/2=4.4° is enough to achieve sufficient suppression of the0^(th)-order. A blazed grating having a blaze angle ba is shown in FIG.3.

[0099] These calculations are based on a converging beam with anegligible focal diameter. For a real beam the blaze angle should besomewhat higher, and the pitch somewhat smaller.

[0100] The influence of the wedge structure on the desired, e.g. EUV,reflection can be calculated by comparing the transmission of the wedgestructure at different positions within a period. The amplitudemodulation depth induced by the wedge shaped structure is:$\begin{matrix}{\chi = {1 - {{\exp ( {- \frac{2a\quad \tan \quad \theta}{\mu}} )}.}}} & (18)\end{matrix}$

[0101] For the structure as given above, this modulation depth is equalto 12.2%. The radiation at the surface of the wedges can be divided intoan oscillating part, and a constant component. The constant component(1−χ) will result in specular reflection. The oscillating component χwill be diffracted and lost. The absorption in the diffractive structureis: $\begin{matrix}{A = {{1 - {\frac{1}{a}{\underset{0}{\int\limits^{a}}{{\exp ( {- \frac{2x\quad \tan \quad \theta}{\mu}} )}d\quad x}}}} = {1 - {{\frac{\mu}{2a\quad \tan \quad \theta}\lbrack {1 - {\exp ( {- \frac{2a\quad \tan \quad \theta}{\mu}} )}} \rbrack}.}}}} & (19)\end{matrix}$

[0102] For the structure as given above, this absorption is equal to6.3%.

[0103] The total specular reflection for EUV radiation of the mirrorincluding the diffractive structure is:

R=R_(ML)(1−χ)·  (20)

[0104] Note that the surface roughness of the diffractive structureshould be low in order to have a low stray light level. However, theroughness criterion is less stringent than for the multilayer mirrorsurface and grazing incidence mirror surfaces. It is comparable to theroughness criterion of EUV filters.

[0105] The diffractive structure can be covered with a cap layer as forinstance ruthenium provided that the thickness of this layer is uniformenough to neglect diffraction by the cap layer itself. The cap layershould be thin enough to pass the desired radiation.

[0106] When necessary, the orientation of the lines of the diffractivestructure with respect to the plane created by the EUV beam can bechosen in a direction different from the direction as shown in FIG. 7.

[0107] The diffractive structure can be produced in the same ways asthat of the fourth embodiment.

[0108] Common to both the fourth and fifth embodiments is that thereflective elements 300, 400, having thereon the diffractive structure,act essentially as an undisturbed reflector for desired radiation butact as a grating for undesired radiation. This is achieved by choosing amaterial for the diffractive structure that has a refractive index tomake it as invisible as possible to the desired radiation.

[0109] While we have described above specific embodiments of theinvention it will be appreciated that the invention may be practicedotherwise than described. The description is not intended to limit theinvention.

1. A lithographic projection apparatus comprising: a radiation system toprovide a projection beam of radiation; a support structure to supportpatterning structure, the patterning structure being constructed andarranged to pattern the projection beam according to a desired pattern;a substrate table to hold a substrate; a projection system to projectthe patterned beam onto a target portion of the substrate, a gratingspectral filter comprised in said radiation system for passing radiationof desired wavelengths to form said projection beam and for deflectingradiation of undesired wavelengths.
 2. Apparatus according to claim 1wherein said grating spectral filter comprises a blazed grating. 3.Apparatus according to claim 2 wherein said grating spectral filter hasa blazing angle less than about 2.5°.
 4. Apparatus according to claim 2wherein said grating spectral filter has a line density in the range offrom 200 to 700 lines per mm.
 5. Apparatus according to claim 1 whereinsaid grating spectral filter is a laminar grating.
 6. Apparatusaccording to claim 1 wherein said grating spectral filter is constructedso as to substantially not affect radiation of said desired wavelengths.7. Apparatus according to claim 6 wherein said grating spectral filteris substantially formed of a material that essentially does not affectradiation of said desired wavelengths.
 8. Apparatus according to claim 7wherein said grating spectral filter is substantially formed of amaterial having a refractive index close to unity at said desiredwavelengths.
 9. Apparatus according to claim 8 wherein said gratingspectral filter comprises silicon.
 10. Apparatus according to claim 1further comprising a cooling element provided in thermal contact withsaid grating spectral filter.
 11. Apparatus according to claim 10wherein said cooling element comprises coolant channels.
 12. Apparatusaccording to claim 11 further comprising a cooling system for passingcoolant fluid through said coolant channels.
 13. Apparatus according toclaim 1 wherein said grating spectral filter is a reflective filter. 14.Apparatus according to claim 1 wherein said grating spectral filter is agrazing incidence reflector.
 15. Apparatus according to claim 13 whereinsaid grating spectral filter is integral with an optical element of saidillumination system.
 16. Apparatus according to claim 1 wherein saidprojection beam comprises extreme ultraviolet radiation
 17. Apparatusaccording to claim 16 wherein the extreme ultraviolet radiation has awavelength in the range of from about 8 mm to about 20 mm.
 18. Apparatusaccording to claim 16 wherein the extreme ultraviolet radiation has awavelength in the range of from about 9 mm to about 16 mm.
 19. Anapparatus according to claim 1 wherein the support structure comprises amask table to hold a mask.
 20. An apparatus according to claim 1,wherein the radiation system comprises a radiation source.
 21. Apparatusaccording to claim 20 wherein said radiation source is a laser-produced,or discharge, plasma radiation source.
 22. A device manufacturing methodcomprising: providing a substrate that is at least partially covered bya layer of radiation-sensitive material; projecting a patterned beam ofradiation onto a target portion of a layer of radiation-sensitivematerial on a substrate; and filtering said projection beam in saidradiation system using a grating spectral filter.
 21. A devicemanufactured in accordance with the method of claim 20.